The present invention relates to photosetting resin compositions whose principal components are photo-polymerizable unsaturated compositions used in the fabrication of printed circuit boards, high-precision metal parts and so on and which may be used as the solder resist ink or the like which is set with ultraviolet radiation after having been applied so as to form a protective coating.
In the fabrication of printed circuit boards, the solder resist ink has been used in order to provide the permanent protection for printed circuits and to prevent solder from being bridged. It follows, therefore, that the qualities of the solder resist ink determine those of the printed circuit boards.
The principal components of the conventional solder resist ink are epoxy resins, melamine resins, urea resins, alkyd resins and their denatured compositions. After being uniformly applied through a screen, the solid ink is set thermally so that hard protective coatings may be formed.
With the conventional solder resist ink, the setting time is from 10 to 20 minutes so that productivity is not satisfactory. In addition, since the solder resist ink contains organic solvents, the atmospheric pollution problems arise. That is, the operators are annoyed with odors of solvents themselves in the printing step and formalin odors due to the formation of formaldehyde in the setting step.
The solder resist ink contains 60 to 80% of nonvolatile matter so that it must be applied relatively thick in order to obtain a desired degree of resistance-to-chemical, heat resistance, electrical characteristics and so on. As a result, the solder resist ink can be applied only with a lesser degree of dimensional accuracy. Furthermore, when the printed circuit boards applied with the solder resist ink are subjected to heating, some properties of the boards are lost. In addition, before the boards are transferred to the next step, they must be cooled. Thus, workability and productivity are substantially reduced.
In order to overcome the above-described problems, there have been proposed various solder resist inks whose principal components are monomers and prepolymers having radical reactive unsaturated double bonds and a photo-polymerization initiator. These inks are advantageous in that they can be set quickly with the radiation of ultraviolet radiations and are safe because they do not contain any solvents. They have been used in the fabrication of rigid boards.
However, the solder resist ink of the type described above may have satisfactory resistance-to-solder but has poor flexural strength. Some solder resist inks have satisfactory flexural strength, but are unsatisfactory in resistance-to-solder, so that such ink cannot be used in the fabrication of flexible printed circuit boards. The flexible printed circuit boards are flexible and thin so that their thermal capacity is low. Therefore, the solder resist ink used in the fabrication of flexible printed circuit boards must have excellent resistance-to-solder, adhesion to copper and polyimide, polyester or epoxy films and a high flexural strength. The flexible printed circuit boards are featured in that they are light in weight, thin and flexible. As a result, they are advantageous in (1) that the effective use of the space in an equipment is possible for wiring layouts, (2) that new circuit components can be designed by making use of the flexibility of the printed circuit boards and (3) that the fabrication and assemblying steps can be simplified so that fabrication costs can be lowered. Therefore, it is expected that the flexible printed circuit boards are increasingly used in a variety of fields. However, the solder resist ink which can be satisfactorily used in the fabrication of such flexible printed circuit boards has not been proposed yet.